Fix sampledata reset LastPostAt when the channels are empty (#9295)

store/sqlstore/channel_store.go

@@ -1924,7 +1924,14 @@ func (s SqlChannelStore) ClearAllCustomRoleAssignments() store.StoreChannel {
 
 func (s SqlChannelStore) ResetLastPostAt() store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
-		if _, err := s.GetMaster().Exec("UPDATE Channels SET LastPostAt = (SELECT UpdateAt FROM Posts WHERE ChannelId = Channels.Id ORDER BY UpdateAt DESC LIMIT 1);"); err != nil {
+		var query string
+		if s.DriverName() == model.DATABASE_DRIVER_POSTGRES {
+			query = "UPDATE Channels SET LastPostAt = COALESCE((SELECT UpdateAt FROM Posts WHERE ChannelId = Channels.Id ORDER BY UpdateAt DESC LIMIT 1), Channels.CreateAt);"
+		} else {
+			query = "UPDATE Channels SET LastPostAt = IFNULL((SELECT UpdateAt FROM Posts WHERE ChannelId = Channels.Id ORDER BY UpdateAt DESC LIMIT 1), Channels.CreateAt);"
+		}
+
+		if _, err := s.GetMaster().Exec(query); err != nil {
 			result.Err = model.NewAppError("SqlChannelStore.ResetLastPostAt", "store.sql_channel.reset_last_post_at.app_error", nil, err.Error(), http.StatusInternalServerError)
 		}
 	})